Engines, including diesel engines, are being designed to operate under lean conditions as a fuel economy measure. Such future engines are referred to as “lean burn engines.” That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are “lean,” i.e., the exhaust gases are relatively high in oxygen content. Although lean-burn engines provide advanced fuel economy, they have the disadvantage that conventional three-way catalytic converters (TWC) are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. Attempts to overcome this problem have included the use of a NOx trap. The exhaust of such engines are treated with a catalyst/NOx sorbent which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich (or stoichiometric) operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with hydrocarbon (HC), carbonmonoxide (CO), and/or hydrogen present in the exhaust.
Diesel engines provide better fuel economy than gasoline engines and normally operate 100% of the time under lean conditions, where the reduction of NOx is difficult due to the presence of excess oxygen. In this case, the catalyst/NOx sorbent is effective for storing NOx. After the NOx storage mode, a transient rich condition must be utilized to release/reduce the stored NOx to nitrogen.
NOx storage (sorbent) components including alkaline earth metal oxides, such as oxides of Mg, Ca, Sr, and Ba, alkali metal oxides such as oxides of Li, Na, K, Rb, and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr, and Nd in combination with platinum group metal catalysts such as platinum dispersed on an alumina support have been used in the purification of exhaust gas from an internal combustion engine. For NOx storage, barium oxide is usually preferred because it forms nitrates at lean engine operation and releases the nitrates relatively easily under rich conditions. However, catalysts that use barium oxide for NOx storage exhibit a problem in practical application, particularly when the catalysts are aged by exposure to high temperatures and lean operating conditions. After such exposure, such catalysts show a marked decrease in catalytic activity for NOx reduction, particularly at low temperature (200 to 350° C.) operating conditions.
In a reducing environment, a lean NOx trap (LNT) activates reactions by promoting a steam reforming reaction of hydrocarbons and a water gas shift (WGS) reaction to provide H2 as a reductant to abate NOx. The water gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen. The presence of ceria in an LNT catalyzes the WGS reaction, improving the LNT's resistance to SO2 deactivation and stabilizing the PGM. NOx storage materials comprising barium (BaCO3) fixed to ceria (CeO2) have been reported, and these NOx materials have exhibited improved thermal aging properties. Ceria, however, suffers from severe sintering upon hydrothermal aging at high temperatures. The sintering not only causes a decrease in low temperature NOx capacity and WGS activity, but also results in the encapsulation of BaCO3 and PGM by the bulk CeO2. Lean NOx traps generate high N2O emissions when the LNT is placed in an underfloor position because N2O formation in the LNT increases with decreasing temperature. Placing the LNT closer to the engine can reduce N2O emissions, which requires high hydrothermal stability. Thus, there is a need for a ceria-containing LNT that is hydrothermally stable.
In addition, the new Diesel Euro6c legislation, scheduled to become effective in 2017, requires NOx conversions under real driving conditions. Thus, to comply with new Diesel Euro6c legislation, the LNT must store NOx under high (motorway) and low (city) temperature conditions. Additionally, the removal of the stored NOx and conversion to N2 at low temperatures is a challenge. However, the LNT DeNOx regeneration of stored NOx under city driving conditions and the aging stability of NOx storage needs to be improved compared to existing LNT catalysts.